In the past, a color image forming apparatus includes an optical scanning device including a lens that allows, with one lens, plural beams corresponding to colors to pass. The optical scanning device includes a set of a first fθ lens and a second fθ lens that use the plural beams in common. In the optical scanning device, light passed through the first fθ lens is made incident on the second fθ lens. After passing through the second fθ lens, the light is decomposed for each of the colors using a reflection mirror and led to a photoconductive member. Therefore, in the optical scanning device, a light source, a polygon mirror, and the first fθ lens are arranged on one side of the second fθ lens. The reflection mirror for separating the colors is arranged on the other side of the second fθ lens. The polygon mirror, the first fθ lens, the second fθ lens, and the reflection mirror for separating the colors provided in the optical scanning device are arranged in one direction. In other words, the polygon mirror, the first fθ lens, the second fθ lens, and the reflection mirror for separating the colors are arranged in an optical axis direction of light reflected by the polygon mirror.
Therefore, the optical scanning device is elongated in the optical axis direction of the light reflected by the polygon mirror. As a result, the size of the color image forming apparatus is increased.